1. Field of the Invention
This invention relates to oil and gas well drilling and more particularly to the removal of suspended solids which are normally present in completion fluids used in these drilling operations.
2. Prior Art
In oil and gas well drilling operations, it is necessary to pump clean completion fluids such as CaCl.sub.2, CaBr.sub.2 or ZnBr.sub.2 into the producing zone in order to clean the formation pores thus enhancing flow from the formation to the well head. These completion fluids are recirculated into the producing zone a sufficient number of time to achieve the desired pore purity. Unfortunately any suspended solids which are present in such fluids can be deposited in the producing formation where they can be trapped by the matrix or in the perforation tunnels of the formation and reduce productivity. It is very difficult to remove these solids once they have been placed into the formation, especially if they have been pumped some distance into the formation matrix from the well bore. Thus, it is desirable to remove these solids prior to initial injection and subsequent recirculation into the well bores.
Currently, the preferred filtering method is the cartridge (sock type or string wound) filter elements made of fiber, paper, cotton, sisal and other materials. These elements are fitted over a perforated core and sealed. The fluid is then pumped through the filter elements and out of the perforated core. However, these systems must use multiple pass filtration through progressively finer filter cartridges. Thus, this filtration system is very time consuming, i.e., average time for completing the cleaning of the formation pores is ten days. Additionally, as reported by Sparlin and Guidry, Study of Filters Used for Filtering Workover Fluids, Society of Petroleum Engineers Paper 7005, 1968, this method of filtration only removes approximately 85% of the suspended solids present in the fluid. The remainder therefore is injected into the producing formation where it can cause damage and reduction of production rates.